Several superconducting devices of today, such as superconducting computers and superconducting magnets of magnetic resonance imaging systems, use an inventory of liquid cryogen (i.e. helium) for continuous refrigeration. Usually a cryostat or vacuum jacketed reservoir of the liquid cryogen is used to cool the device to achieve superconductivity. As the device is used, heat is generated and the inventory of liquid cryogen boils off. In the case of mobile magnetic resonance imaging systems, it is necessary to demagnetize the device for each road trip. The demagnetization process further causes several liters of cryogen to be boiled off. In order to maintain and replenish the inventory of liquid cryogen a continuous supply of gaseous cryogen must be provided, liquified and introduced into the liquid inventory; or a means of recondensing the boil off back into the liquid inventory must be provided.
One approach to recondensation has been to collect the venting gas and direct it to refrigeration apparatus outside of the cryostat which recondenses the cryogen. The liquid cryogen is reintroduced into the cryostat. However, problems arise in transferring the liquid cryogen back to the cryostat while maintaining the cold temperature.
Another approach has been to place a refrigerator directly in an access port or neck of the cryostat. Such refrigerators are disclosed in U.S. Pat. Nos. 4,223,540 and 4,484,458. Each discloses a displacer-expander refrigerator in conjunction with a Joule-Thomson heat exchanger. The refrigerator is disposed in at least one access port to cool heat shields of the cryostat and to recondense the cryogen boil-off. U.S. Pat. No. 4,223,540 minimizes heat transfer losses by matching the temperature gradient in the access port. U.S. Pat. No. 4,484,458 matches the thermal gradient in the heat exchanger with that of the refrigerator, to minimize heat loss in the cryostat when the refrigerator is in use.
Having the apparatus or a refrigerator disposed within the cryostat housing, it then becomes necessary to provide means to remove the refrigerator should it have to be serviced. With such removal, however, there is a danger of exposing the liquid cryogen inventory to ambient conditions and allowing heat infiltration which would in turn promote cryogen boil-off. One method to solve this problem of removal is to specially design the cryostat. However, the refrigerators for such cryostats typically have relatively high heat transfer losses, and the cryostats have large cross-sectional areas. U.S. Pat. No. 4,223,540 discloses a cryostat utilizing a closed-cycle refrigerator with several stages of refrigeration to intercept heat leak into the liquid cryogen and to recondense cryogen boil-off. The cryostat is adapted to removal, repair and replacement of the refrigerator while the superconducting device continues operation. However, designing such a cryostat for each different super conducting device is costly and impractical.
A further problem with cryostat refrigerators of prior art is the large access area to the cryostat necessitated by the refrigerator compared to the smaller access ports of todays devices. Smaller access ports are being made to decrease the amount of heat infiltration to the cryogen and therefore to prevent promotion of boil-off. More particularly, in the case of a magnetic resonance imaging system, the access port is about one inch in diameter which is much smaller in diameter than any refrigerator of prior art.
In another approach, it has been suggested to condense an outside source of helium gas to liquid form, transfer the liquid helium into a cryostat through a transfer line in heat exchange with the boil-off and thereby recondense the boil off to replenish the liquid cryogen contained in the cryostat.